Nonaqueous electrolyte secondary cells are widely used for power supplies for mobile devices, such as laptop computers or mobile phones. Furthermore, recently, nonaqueous electrolyte secondary cells have been installed in electric cars or the like, and are gaining attention in new fields. The automotive field seeks to achieve higher energy density in secondary cells, especially for improving cruising distance, which is regarded as important to the performance required for nonaqueous electrolyte secondary cells.
Generally, anodes made of graphite-based active materials are used as anodes for nonaqueous electrolyte secondary cells. The theoretical capacity of graphite is 372 mAh per gram of active material. In contrast, recently, silicon (Si) or tin (Sn) is drawing attention as an active material that has a capacity exceeding graphite. The theoretical capacity of silicon is 4200 mAh/g per 1 g of an active material, and similarly, that of Sn is 990 mAh/g per 1 g of an active material.
Since silicon has approximately 11 times the capacity of graphite, the volume change associated with occlusion and release of lithium ions is large and thus the volume increases by a factor of approximately 4 due to lithium occlusion. Compared to graphite, an electrode using an active material having a large capacity experiences a large volume change during charging and discharging. Therefore, there are risks that the conductive path of the electrode may be disconnected, or the active material may be separated from the electrode due to pulverization, or there may be a peeling between the collector and the active material layer. These could be factors of deterioration in the cycle characteristics of secondary cells. Accordingly, the development of an anode is underway, which uses a metal oxide with a smaller discharge capacity and with a smaller volume expansion compared to the aforementioned metal based material. However, problems similar to the above are still caused even when such a metal oxide-based active material is used, and the problems have not yet been completely solved.
Accordingly, PTL 1, for example, proposes an electrode using compound active material particles in which the surfaces of silicon particles are each covered with a carbon layer. However, use of only the covering layer is not sufficient to suppress pulverization due to the large volume change in the simple substance of alloy-based active material particles.
To avoid disconnection of the conductive path of the electrode or separation of the active material conductive path from the electrode due to pulverization, or peeling between the collector and the active material layer, for example, PTL 2 proposes that the surfaces of the silicon particles are treated with a silane coupling agent having a specific functional group (a functional group having π electrons or a functional group containing an element having a lone pair), to produce a strong interaction between a conductive assistant, such as carbon, and a metal foil collector. However, PTL 2 does not teach the type of the binder. To improve cycle characteristics, the binding strength is required to be enhanced.
In PTL 3, an alginic acid having a good affinity for a carbon-based active material and a conductive assistant are used as a binder for an electrode to prevent peeling of the anode active material or the conductive assistant and thus improving the cycle characteristics. However, since a silicon-based active material is different from a carbon-based active material in the surface conditions and physical properties, the configuration described in PTL 3 cannot be applied as it is.
PTL 4, for example, proposes a method of improving cycle characteristics of an anode containing silicon. In the method, a surface-treated silicon-based active material is mixed with a binder, followed by heat treatment, and dehydration and condensation, to increase binding strength and to thereby prevent separation of the active material from the electrode. In PTL 5, an electrode is formed by providing a coating of an anode active material slurry, to which polycarboxylic acid or polyamine added has been added, followed by heating, to increase binding strength and to thereby prevent separation of the active material from the electrode. However, a covalent binding established between a binder and an active material by heat treatment, cannot be recovered once the binding is cut due to the change in the volume of the active material associated with charging and discharging. This leads to a problem of deteriorating cycle characteristics.